1. Field of the Invention
The present invention relates to methods and apparatus for pollution control, and is particularly directed to controlling atmospheric emissions from cement plants.
2. Background
Over the past several decades air pollution control has been a priority concern of society. In the United States primary regulatory authority over industrial source air emissions resides in the U.S. Environmental Protection Agency (“EPA”). Over the years, the EPA has increased the stringency of its air pollution control programs, both by decreasing the limits on acceptable emissions and by continually increasing the number and types of regulated pollutants. The regulatory approach has been to require sources of air pollution to adopt the best available control technologies (“BACT”). In some instances, particularly where potentially toxic compounds are released into the atmosphere, sources are required to use the maximum available control technology (“MACT”).
The foregoing control programs are applicable to cement manufacturing facilities, including those which use a raw mill to grind the raw materials (“feed meal”) before it is fed into a pyroprocessing kiln. Two well known types of devices to remove common particulates from the gaseous effluent stream of a cement plant are electrostatic precipitators (ESPs) and fabric filter baghouse (FFB) collectors. ESPs are generally recognized as being capable of a high particle collection efficiency of fine particles when the particles have the proper electrical resistivity. FFBs are also generally recognized as being capable of a high particle collection efficiency of fine particles when the particles have the proper characteristics. Typically, the effluent gas stream is “conditioned” prior to discharge such that the particulates can be more efficiently filtered by ESPs or FFBs. However, ESPs and FFBs are designed to remove particulates and do not remove many gaseous pollutants. Certain species of acid-forming gases can penetrate conventional particulate collection devices such as ESPs or FFBs resulting in emissions violations, causing downstream corrosion of components, and contributing to visible condensed particles which form after exiting the stack. In particular, ESPs and FFBs do not remove sulfur dioxide and other gases that form acids when dissolved in water.
The release of acid-forming gases from a cement plant may violate pollution control standards, may contribute to the “detached plume” phenomena and may contribute to acid rain. Acid forming gases are now known to contribute to the formation of visible plumes of effluent that violate opacity regulations even when the total quantity of acid forming gases released into the atmosphere is comparatively minor. For example, one air pollution control problem for cement plants is the formation of a detached plume. Experimental studies have identified the detached plumes as being comprised primarily of ammonium sulfate and ammonium chloride particulates that form and condense as the emissions from the stack cool in the atmosphere a distance from the stack. The ammonium particulates are in a size range of approximately one micron, which is a size that is efficient at scattering and reflecting light. The small size of the particulates and their high scattering efficiency means that an optically opaque plume can be comprised of a comparatively small total mass of ammonium sulfate and ammonium chloride particulates. These detached plumes consist of a white plume of fine particles that may last for hours or days depending on plant and atmospheric conditions. The plumes are highly noticeable and may violate pollution control regulations for opacity, and are considered a potentially serious problem.
One solution to this problem would be to add a wet chemical scrubber immediately after the ESP or FFB. However, conventional methods to scrub acid forming gases are typically expensive and inconsistent with the economic operation of an energy efficient cement plant. For example, conventional wet scrubbers, which commonly use spray droplet sizes greater than 1000 microns, typically use 10-100 gallons per minute of scrubbing liquid to scrub 1000 standard cubic feet per minute of effluent gases (1-10 kilograms of liquid per kilogram of gas). Consequently, the consumption of water, scrubbing chemicals, and energy would be quite large for a conventional liquid scrubber.
The hot cement kiln gases must be cooled to approximately 150° C. to have acceptable particulate emissions from an ESP or cooled to approximately 180° C. to protect a FFB from overheating. Modern cement plants commonly use two methods to cool and condition hot process gases before they enter an ESP or FFB. A gas conditioning tower (“GCT”) uses a spray of water to cool and condition the gaseous effluent. Further cooling and conditioning may then be performed by passing the gaseous effluent through the cool wet limestone of a raw mill that is used to grind the raw materials into a “feed meal,” which is then transported to the pyroprocessing kiln. However, cooling in the raw mill only occurs when the raw mill is operational. When the raw mill is on, the gas conditioning tower typically must only cool the gaseous effluent to around 250° C. After it leaves the GCT, in the mill-on state, the effluent is then further cooled to between 90° C. and 140° C. as it passes through the raw mill. However, when the raw mill is off, the cooling tower must provide all of the necessary cooling. There are thus two distinct operational states of the cement plant, corresponding to a mill-on and a mill-off condition.
One prior art solution to the problem of sulfur dioxide emissions from a cement plant, as disclosed in the inventor's prior U.S. Pat. No. 6,464,952, the disclosure of which is incorporated by reference, is to inject a high-pH lime slurry into the spray used in the GCT. As described therein, while other high pH materials can be used, a lime slurry is a desirable scrubbing material because lime is chemically compatible with other chemical constituents of cement, i.e., the cement will not be deleteriously contaminated if small quantities of lime enter the feed meal subsequent to the GCT. The lime slurry reacts with sulfur dioxide and other acid forming gases to produce thermally stable salts, thereby reducing emissions of acid forming gases.
Nonetheless further methods and apparatuses for reducing acid forming gases to be efficiently and economically scrubbed are desired.